I somewhat disagree with OP. I think in maybe 10-15 years the classic EMs will start to become like woodrails are now. Interesting only to the oldtimers and hardcore collectors. Most pinheads will not own one. If it's not a desirable title you'll have trouble giving it away. In collections they'll be there just as a curiosity.

I think part of the value increase is due not only to supply and demand of games but also availability of parts. It's pretty incredible the amount of reproduction parts available for these things nowadays. Especially with all the reproduction playfields and backglasses out there now. Of course that cuts both ways, a game with repro parts available becomes more valuable, and rising game prices also makes reproduction business viable.